1. Field of the Invention
This invention relates to a pneumatic tire having an electrically conducting path formed by using an electrically conductive rubber composition. More particularly, it relates to a pneumatic tire having a low conductive tread compounded with a great amount of a filler such as silica for the improvement of low fuel consumption in which an electrically conducting path is formed in the tread by using an electrically conductive rubber composition for the prevention of electricity.
2. Description of Related Art
The pneumatic tire provided with a tread having a lower fuel consumption, particularly a silica-containing tread is high in electric resistance and low in the electric conductivity, so that static electricity generated from a vehicle body or a tire is hardly dissipated to ground surface through the tread. Hence there are problems such as noise to radio, electric shock, spark and the like.
There are known the following methods for solving the above problems.
As a first method, an electrically conductive rubber thick sheet is extended in a central portion of a tread in a widthwise direction thereof from an outer surface of the tread to a base rubber of the tread (see EP 0 658 452B1 and JP-A-8-34204), or an electrically conductive rubber thin sheet is extended from a shoulder portion of the tread to an inside of a sidewall (see U.S. Pat. No. 5,518,055). Also an electrically conductive film is arranged on only the outer surface of the tread as a whole (see EP 0 718 126A2). Furthermore, EP 0 718 126A1 discloses that an electrically conductive member is disposed beneath the tread and a projecting extension is formed to extend from such an electrically conductive member toward an outer surface of the tread. As to the former case, there is a description that the projecting extension is continuously extended in a circumferential direction of the tread at a thick thickness.
As a second method, carbon black different from that usually used in the tire and having an excellent electric conductivity is compounded with a tread rubber.
As a third method, there is a method of applying onto a surface of a tread an electrically conductive substance such as a water-based electrically conducting rubber cement containing an electrically conductive carbon black or the like during the extrusion of the tread in the production of the tire (e.g. see JP-A-8-120120). According to this method, even if a product tire after vulcanization is mounted onto a passenger car and run over a long time to wear a ground contact region of the tread, the electrically conducting substance still remains in side walls of many grooves forming a tread pattern of the ground contact region, which can dissipate static electricity charged in the entirety of the tire to ground surface.
However, all of these methods have problems in the production and quality as mentioned below and are not necessarily satisfied.
For example, the antistatic effect is maintained at an initial running stage when the electrically conductive thick rubber layer is arranged in a central portion of the tread in its widthwise direction from the surface of the tread up to a rubber beneath the tread as disclosed in EP 0 658 452 and JP-A-8-34204, but there is a problem that if general-purpose carbon black is used as a filler, the electrically conducting path is interrupted at the running end stage due to the promotion of wearing of the electrically conductive layer to eliminate the antistatic effect. Particularly, in order to maintain the effect of improving the wear resistance of the tread rubber or a tread cap rubber, the wear resistance of the electrically conductive rubber layer should be improved like the tread cap rubber, but it turns out that only the tread cap rubber actually contacts the ground and hence the antistatic effect can not be obtained.
Further, when several parts by weight of the electrically conductive carbon black based on 100 parts by weight of rubber ingredient is added to the tread rubber for the tire, the electric resistance of the tread rubber is lowered, but the low fuel consumption as a primary object of the tire is considerably degraded. Also, the carbon black itself is considerably low in the reinforcing effect for the polymer and hence there is caused a problem of lowering the wear resistance of the tire tread.
In the method of applying the water-based rubber cement containing the electrically conductive carbon black onto the surface of the tread cap rubber, there is a problem in the stationary stability of the cement itself and hence there is a fear of causing phase separation. Furthermore, it is required to use various stabilizers in order to prevent the blowing in the application, which degrades the durability of the resulting rubber cement film after the vulcanization and also results in the contamination of the mold for vulcanization. Moreover, the rubber composition used in the tread cap rubber is hydrophobic, so that when the above water-based rubber cement is applied onto the tread cap rubber, drying takes a long time and the application unevenness is caused and hence the durability of the applied film is degraded. Also, the adhesion force at the boundary between the tread cap rubber and the film of the water-based rubber cement lowers and hence boundary peeling is caused during the running of the tire, so that there is caused a problem that the antistatic effect is not obtained at the end stage of tire running due to the breakage of electrically conducting path.
Moreover when the electrically conductive rubber layer is disposed on a shoulder portion of the tread as disclosed in U.S. Pat. No. 5,518,055 and EP 0 718 126, the effect is developed at an initial running stage, but the rubber layer is rapidly removed from a ground contact region due to the wearing to lose the effect. That is, the shoulder portion is prematurely worn due to uneven wear in the tire as compared with the central portion of the tread to form a step difference between the central portion and the shoulder portion and hence the shoulder portion is not necessarily contacted with ground during the usual running. Therefore, even if the electrically conductive rubber layer remains in a zone ranging from a tread end toward a sidewall portion, it is difficult to provide an antistatic effect because the rubber portion is already removed from the tread portion.
On the other hand, when the electrically conductive thin sheet is disposed on the groove, the effect is developed at an initial running stage, but the neighborhood of the groove edge is rapidly worn during the running and hence the sheet is not always contacted with ground. Further, it has been confirmed that the electrically conductive rubber sheet is cut out by stones entered in the groove during the running and is not often retained in the groove on and after a middle running stage.
It is, therefore, an object of the invention to provide a low fuel consumption pneumatic tire comprising an electrically low conductive tread compounded with a great amount of a filler such as silica and having considerably improved antistatic effect and stationary stability.
It is another object of the invention to maintain the higher antistatic effect in the above pneumatic tire over a period ranging from the initial running stage up to the middle running stage and after thereof.
The inventors have made various studies in order to solve the aforementioned problems and found that the above object is achieved by applying a rubber layer having a particular specific resistance to a given place of a low fuel consumption pneumatic tire comprising an electrically low conductive tread to form an electrically conducting path and as a result the invention has been accomplished.
According to the invention, there is the provision of a pneumatic tire comprising a tread of a cap and base structure and at least one electrically conductive rubber layer having a specific resistance of not more than 106 xcexa9xc2x7cm and extending in a circumferential direction of the tread. The electrically conductive rubber layer has a thickness of 0.1-1 mm and extends inward from an outer surface of the tread in a radial direction and contacts with a tread base rubber having a specific resistance of not more than 108 xcexa9xc2x7cm adjacent to an inside of a tread cap rubber having a specific resistance of more than 108 xcexa9xc2x7cm. The electrically conductive rubber layer is disposed in a region corresponding to 70% of a ground contact width of a new tire and in a land portion defined between circumferential grooves located in the region so as to contact both sides of the rubber layer in a widthwise direction thereof with the tread cap rubber. The electrically conductive rubber layer is made from a rubber composition containing 40-100 parts by weight of carbon black having a nitrogen adsorption specific surface area (N2SA) of 130-150 m2/g and a dibutyl phthalate absorption (DBP) of 110-135 ml/100 g based on 100 parts by weight of a diene rubber ingredient.
In a preferable embodiment of the invention, only one electrically conductive rubber layer is disposed near to the equatorial plane of the tire in a region corresponding to 50% of the ground contact width of the new tire.
In another preferable embodiment of the invention, the tread base rubber has a tan xcex4 at 60xc2x0 C. smaller than that of the tread cap rubber, and the tan xcex4 at 60xc2x0 C. of the tread base rubber is not more than 0.14.
In the other preferable embodiment of the invention, the tread base rubber has a thickness of 1.5-3.0 mm.